Field of the Invention
This invention relates, in general, to material fastening methods and, in particular, to a new and useful method and apparatus for connecting together material plies which are unilaterally coated with a thermoplastic material.
Water-proof articles of clothing for bad weather conditions, work- or sporting activity, are usually made of substrate tissues unilaterally coated with a thermoplastic material, so that the material does not harm the user's skin, especially preventing water condensation inside the clothing. In the manufacture of water-proofed canvass for tents or other protection, cost considerations are the reason for employing such coated tissues.
Several methods are known for connecting to each other coated tissue plies during manufacture.
First, the connection by sewing. A sewn seam does provide a firm and durable connection, however, the greatest advantage of the coated tissue, namely the absolute waterproofness, is lost and the seam must subsequently be sealed in a time-consuming operation hardly satisfactory in its effect, for example covered with a hot-sealed strip.
Another connection is by hot-wedge high-frequency or hot-gas sealing, by which the coated sides of the materials are bonded to each other. Even though this is a continuous process, it is relatively seldom applied, since the strength of the bond is limited to the adhesion of the coating to the underlying tissue and the requirements in the various applications are mostly stricter. Moreover, in this kind of connection, the cut edges of the tissue plies are not secured against fraying.
In another method disclosed in U.S. Pat. No. 3,172,798 a connection is produced by first sewing together the two tissue plies and a thermoplastic strip, superposed in this order, and then folding the upper ply about the thermoplastic strip back through 180.degree., so that the strip comes to extend within the fold thus formed. Thereupon, the superposed layers are heat-sealed to each other in a high-frequency welding process. In this process, the work must be positioned and heat-sealed by sections. The operation thus is cyclical and rather time-consuming. In addition, heat-sealing of certain curved portions either requires a change of tools, or permits only relatively short tools. Since under high loads, the thread works in the perforations made by the needle, leaks may occur in spite of the high-frequency sealing.
Finally, a method is known from German AS No. 20 33 830 providing inserting a strip of thermoplastic material between the overlapping marginal portions to be connected to each other of two coated material plies, so as to protrude on both sides beyond the two end edges of the plies, and sewing the plies and the strip with one or more seams together. One or both of the protruding edge portions of the strip are then doubled back over the adjacent end edges of the sewn composite structure and hot-sealed by high frequency welding to the respective coated ply or plies.
The connection obtained with this prior art method is of good quality and may certainly be durable, however, it is not suitable for articles such as of clothing because it produces considerably thickened portions in the material webs, which are not sufficiently flexible and supple.